1. Field of the Invention
The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly and separators alternately. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
Some of the fuel cells of this type are designed to use the entire surfaces of unit cells for power generation. For example, Japanese Laid-Open Patent Publication No. 2002-203579 discloses a solid oxide fuel cell shown in FIG. 21. As shown FIG. 21, the solid oxide fuel cell is formed by stacking power generation cells 1 and separators 2 alternately. Each of the power generation cells 1 includes a fuel electrode layer 1b, an air electrode layer 1c, and a solid electrolyte layer 1a interposed between the fuel electrode layer 1b and the air electrode layer 1c. A porous conductive fuel electrode current collector 3 is provided on one surface of the power generation cell 1, and a porous conductive air electrode current collector 4 is provided on the other surface of the power generation cell 1. The fuel electrode current collector 3, the power generation cell 1, and the air electrode current collector 4 are sandwiched between a pair of separators 2.
The separator 2 has a fuel gas supply passage 5 and an air supply passage 6. The fuel gas supply passage 5 is connected to a fuel gas hole 5a formed at a substantially central region on one surface of the separator 2. The air supply passage 6 is connected to an air hole 6a formed at a substantially central region on the other surface of the separator 2. The fuel gas hole 5a faces the fuel electrode current collector 3. The air hole 6a faces the air electrode current collector 4.
The fuel gas such as H2 or CO flows through the fuel gas supply passage 5, and is discharged from the substantially central region of the separator 2 toward the center of the fuel electrode current collector 3. The fuel gas flows through holes formed in the fuel electrode current collector 3 toward the substantially central region of the fuel electrode layer 1b. Then, the fuel gas flows along unillustrated slits to move radially outwardly toward the outer region of the fuel electrode layer 1b. 
Likewise, the air is supplied from the substantially central region of the separator 2 toward the center of the air electrode current collector 4 through the air supply passage 6. The air flows through holes formed in the air electrode current collector 4 toward the substantially central region of the air electrode layer 1c. Then, the air flows along unillustrated slits to move radially outwardly toward the outer region of the air electrode layer 1c. In this manner, in each of the power generation cells 1, the fuel gas is supplied to the surface of the fuel electrode layer 1b, and the air is supplied to the surface of the air electrode layer 1c to carry out power generation.
Further, for example, Japanese Laid-Open Patent Publication No. 2002-313370 discloses a separator for a solid oxide fuel cell. At least one air supply through hole extends through an outer region to a central region of the separator. The air supply through hole has an opening at the central region of the separator. Further, at least one fuel gas supply hole extends through an outer region to a central region of a separator. The fuel gas supply hole has an opening at the central region of the separator. The opening of the air supply hole and the opening of the fuel gas supply hole are formed on opposite surfaces of the separator.
According to the disclosure of Japanese Laid-Open Patent Publication No. 2002-203579, the fuel gas flows outwardly from the substantially central region to the outer region of the fuel electrode layer 1b, and the air flows outwardly from the substantially central region to the outer region of the air electrode layer 1c. At this time, the gas discharged to the outside of the air electrode layer 1c may enter the fuel electrode layer 1b undesirably. Thus, oxygen component in the gas may react with the fuel gas, and the fuel gas is consumed. Consequently, the power generation performance of the fuel gas is lowered.
Further, for example, when the fuel gas flows outwardly from the substantially central region to the outer region of the fuel electrode layer 1b, the fuel gas is consumed. Therefore, in the outer region of the fuel electrode layer 1b, concentration of the fuel gas is reduced. As a result, the power generation performance in the outer region of the fuel electrode layer 1b is low in comparison with power generation performance in the substantially central region of the fuel electrode layer 1b. The same problems occur also in Japanese Laid-Open Patent Publication No. 2002-313370.